Bacteria of the genus Chlamydia (psittacosis-trachoma organisms) will be grown in cultures of mouse fibroblasts (L cells). These problems will be studied: 1. Isolation and characterization of conditional lethal mutants of C. psittaci. 2. Mechanism of inhibition of DNA synthesis in L cells infected with C. psittaci and C. trachomatis: relation to the death of infected host cells in culture and to the toxic death of mice injected with chlamydiae. 3. Isolation of intact inclusions of C. trachomatis: characterization of inclusion membranes and of membrane-bound chlamydial cells. 4. Further development of experimental models for studying low-grade and latent chlamydial infections.